Globally, river and coastal flooding are the most frequent natural disasters. Technically, a flood is the inundation of a dry land because of the overflowing of a large quantity of water. Floods occur naturally following heavy rains, storms or monsoons. Tsunamis and high tides can also cause floods in coastal areas. We distinguish between two major types of floods: “flash-floods” where heavy downpours in higher grounds produce raging torrents of water reaching areas located downriver in a very short time. “Slow-onset floods” occur when the water run-off reaches flat floodplain areas and wide lowland coastal areas: it takes time for the water to inundate these terrains.

Throughout the world, floods increase in frequency and severity. Some incriminate weather changes and sea level rising. However the reason behind this surge is directly related to human activities, such as (1) large scale deforestation and forest conversion to other types of land uses that prevent the forest from buffering the impacts of heavy downpours (see below); (2) unregulated construction of cities, roads and bridges in geo-sensitive areas prone to landslides (steep slopes) and on natural floodplains used by excess water in case of flooding; (3) indiscriminate building of hydroelectric dams and other structures altering the natural hydrological cycle and course of the river; (4) conversion of peatlands (see below) and other terrains that retain vast quantities of water and regulate hydrological cycles, etc. Poor land-use practices and continuing environmental degradation are responsible for most flooding events in the world today.

•Relationships between deforestation and flooding

Relationships between flooding and land use is complex and depend primarily on how soils and vegetation change through time1. The current understanding between flooding and land use change in the tropics can be summarized as follow:

•Land use changes influence rainfall-induced floods through changes in water interception processes (direct evaporation by the forest), infiltration into the soil and possibilities of water retention by the soil;

•Forest degradation and conversion increase the speed at which rainwater runs off into the river by decreasing infiltration rates into the soil;

•Constructions and roads, added to soil compaction modify the paths and speed of water movements, often providing direct ways for run off into streams, which will increase peak volumes.

Deforestation and conversion of wetlands to intensive land uses (mining, croplands) and infrastructure (roads, cities, etc.) will increase the frequency and severity of floods, at least for those of moderate magnitude that are the most frequent.

Studies on Borneo clearly indicate that when peatswamp forests are converted to other types of land-uses, they dry out, decompose, subside and disappear below sea-level. Then these peat areas become prone to major flooding events.

In Sarawak, drainage of the peatlands located in the Rajang Delta to cultivate oil palm is causing massive subsidence and will result in massive floods in the coming years. A recent study by Wetland International shows that 42 per cent of the 850,000 hectares of coastal peatland would experience flooding in 25 years, rising to around 82 per cent in 100 years. So business will get some profit for a few decades before the peat areas subside and become permanently flooded and unproductive.

In fact, a quick estimate for Indonesia, that is experiencing the same pattern of peat destruction, indicates up to 10 percent of its productive coastal land area could be lost because of flooding in the same way as the Rajang Delta, affecting millions of people who depend on these ecosystems to survive

•Floods in Borneo

Borneo is one of the most important areas in the world for biodiversity3. Its tropical rainforests, wetlands and river systems harbor a large diversity of terrestrial and aquatic species, including global conservation icons, such as the orangutan, the proboscis monkeys, the Sumatran rhinoceros, the Bornean elephant and etc. However, Borneo has one of the highest rates of deforestation and degradation in the world4.

Borneo receives high levels of rainfall, varying from 1520 – 4820 mm per year across the island, while monthly rainfall varies between 80–310 mm for the driest month and 160–740 mm for the wettest month each year (data between 1950 and 2000). Floods are a natural part of Borneo’s ecological functioning, as indicated by observations and geographical studies in the 19th and early 20th century prior to human modification of major river basins. An overview of disasters that occurred in Southeast Borneo between 1747 and 1891 listed 40 years (28% of the total) in which an over-abundance of rain and flooding had severe impacts on food supplies or caused epidemic disease outbreaks, second only to the impact of small-pox outbreaks5. Similar descriptions are available for various part of what is now West Kalimantan, where in the late 19th century flooding events were strongly correlated to outbreaks of malaria and other epidemic diseases.

According to the recent projections of the Intergovernmental Panel on Climate Change, flooding hazards are going to increase primarily through acceleration of the water cycle and shift in seasonal rain patterns, leading to higher frequencies of extreme rainfall. Tropical regions are especially sensitive to the amplification of extreme rain patterns. More than 30 major rivers flow through Borneo. Disturbances to the hydro-climatic systems, ecosystems and land use in the catchment areas of these rivers will have serious consequences for the island’s water supplies, transportation networks, and the capacity of its people to further develop their economies and moderate the impacts of droughts and fires.

Although our current knowledge of flood events in Borneo is still fragmented and highly incomplete, we can expect the impacts of flooding to increase with growing human alterations of major river basins, the intensification of water cycles and extreme rainfall related to climate change, and with the increase of human populations in coastal and flood prone areas.

For example, in late 2014-early 2015, the worst floods in decades created havoc and serious economic losses in the two Malaysian states of Sabah and Sarawak, as well as in many parts of Kalimantan.

WHAT BORNEO FUTURES RECENT STUDY SHOW?

The United Nations Environment Program supported a recent study titled “Forests, Floods, People and Wildlife on Borneo”6 conducted by Borneo Futures. This study showed that problems caused by flooding in Kalimantan were much larger than previously recognized, that flood risks were being exacerbated by modification of climate land use and urbanization, and that urgent and forward-thinking actions were needed to address these issues.

Our studies showed that by not protecting watershed and riverine forests, floods were increasing both in frequency and severity in much of Borneo. Up to 1.5 million people and 360,000 houses were affected by floods in Kalimantan between 2010 and 2013, or more than 10 percent of the total population. Increases in flood frequencies were primarily concentrated in the middle Mahakam area in East Kalimantan, the lower and middle reaches of the Barito, Kahayan, Sampit and Lamandau Rivers in South and Central Kalimantan, and the low-lying swamps around the Kapuas River in West Kalimantan. These are all areas with high human population densities and increasing agricultural developments, indicating that future economic impacts of flooding could be larger still. That is in line with Malaysian government records that estimate that 4.82 million people (22 percent of the total country’s population) are affected by flooding annually, causing some $250 million in damage every year.

Our data strongly indicated that increases in flooding were most likely in areas that had experienced more extensive deforestation for oil palm development, or severe degradation through logging and fires.

Reforestation or restoring water courses may improve hydrological services, depending on whether soil properties recover, and on the influences of river bed morphology, roads and impervious surfaces in the catchment. However these are lengthy and expensive processes!

6Wells, J., Meijaard, E., Abram, N., Wich, S. 2013. Forests, floods, people and wildlife on Borneo: A review of flooding and analysis of local perceptions of flooding frequencies and trends, and the roles of forests and deforestation in flood regimes, with a view to informing government decision-making on flood monitoring, forest management and biodiversity conservation. UNEP Report, Nairobi, 54 pp.